1. Field of the Invention
The present invention relates to a mask substrate such as a mask blank or a photo mask and its manufacturing method.
2. Description of the Related Art
As semiconductor devices have become finer, it has been desirable to allow a photolithography step to deal with these finer semiconductor devices. The device design rule has already specified a design rule of 0.13 μm. In this case, pattern size accuracy to be controlled is about 10 nm and is thus very demanding.
Since such a very tight accuracy is required, problems with the photolithography step used in a semiconductor manufacturing process are becoming noticeable. One of these problems is a defect in a photo mask substrate (photo mask) on which a pattern is formed and which is used for the photolithography step. This problem will further be described below.
The photo mask is required to have substantially no defects. Thus, a defect inspecting apparatus is used to inspect the photo mask so that a defect found through the inspection can be repaired.
However, such defect inspection and repair require much expense and time. This is one of the major factor that sharply increases the price of the photo mask. Furthermore, only fine defects can be repaired. If there is any defect that significantly disturbs a pattern, a photo mask must be reproduced. This is another factor that sharply increases the price of the photo mask.
Many of such defects found during a process of manufacturing a photo mask are caused by a contaminant already present on the photo mask substrate (mask blank) before a pattern is formed.